The horse lentivirus, equine infectious anemia virus (EIAV), is a retrovirus closely related to the human immunodeficiency virus. EIAV is useful as a model for the study of retroviral expression and latency. A. The influence of the virus-encoded, trans-activating protein, Tat, on EIAV LTR activity continues to be investigated. We have shown that EIAV Tat is able to upregulate EIAV gene expression in a number of some, but not all cell lines. Additionally, not all cell lines which support EIAV transactivation support HIV transactivation. Regions within the EIAV LTR and Tat peptide which are important in transactivation have been identified. In cells highly permissive for EIAV expression, the EIAV R sequences which contain the Tat binding site, TAR, were the only EIAV specific sequences required; HIV enhancer/promoter sequences were able to substitute for analogous sequences within EIAV despite the virtual absence of sequence homology. In contrast, both EIAV and HIV enhancer sequences were required for maximal transactivation in cells restricted for EIAV expression. This work has been published and no further work on this project in ongoing ata this time. As an extension of these studies, mapping of LTR sequences required for basal and transactivated levels of expression in tissue culture cells and primary horse macrophages are ongoing. We recently published work in which EIAV enhancer sequences important for virus expression in primary macrophages were characterized. A protein that interacts with these enhancer sequences and appears to be regulate EIAV expression was identified. In addition, the enhancer sequences and the proteins which bind to those sequences in non- permissive and permissive tissue culture cell lines is being explored. At the present time, these studies involve electrophoretic gel shifting assays as well as transient transfectioun assays for detection of viral gene expression. B. Latency and viral reactivation in an field infected, EIAV-seropositive mare is being investigated. This mare has never been clinically ill with EIAV. No infectious virus has ever been isolated from her, however, by immunocompromising the mare EIAV sequences have been detected by PCR in her peripheral blood monocytes. Further characterization of the latent state of EIAV was tested by passage of whole blood from this mare into a naive animal. This animal seroconverted, but did not become acutely ill. The field-infected mare has now been donated to Washington State University for an ongoing collaborative effort to obtain virus and/or viral sequence data from this latently infected horse. Such studies should provide both viral sequence information as well as information on virus location within a latently infected host.